1. Field of the Invention
The present invention relates to an ink composition favorably used for inkjet recording, an inkjet recording method, a printed material produced therewith, a planographic printing plate produced using the ink composition, and a method for producing a planographic printing plate
2. Description of the Related Art
Cyclic ether compounds such as 3-membered rings and 4-membered rings are known to exhibit high reactivity, and are used as polymerizable compounds contained in curable compositions subjected to photocationic polymerization or heat polymerization using an acid anhydride (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 11-43540 and 11-60702.)
There are many image-recording methods of forming an image on a recording medium such as paper based on image data signals, including those in electrophotographic process, sublimation or fusion heat-transfer process, and inkjet process. Among them, the inkjet process is advantageous in that it allow printing in a cheaper device at a lower running cost, because it forms an image directly on a recording medium by ejecting ink only in desirable image region and thus uses the ink more efficiently. In addition, the inkjet process is also less noisy and thus advantageous as an image-recording method. The inkjet process allows printing not only on plain paper but also on non-water absorbing recording media such as a plastic sheet or metal plate, but is imperatively required to increase the printing speed and improve the image quality. The time required for drying and curing the ink droplets after printing significantly influences the efficiency in producing printed materials and sharpness of the printed image.
In the inkjet recoding processes, there is a recording method by using an inkjet recording ink that cures by irradiation with radiation ray. In the method, it is possible to improve the printing efficiency and the quality of image, by curing the ink droplet by irradiating it with radiation ray immediately or after a particular period from ejection. By increasing the sensitivity of the inkjet recording ink which is curable by irradiation with a radiation ray such as ultraviolet light, the ink has higher curability for radiation rays, which brings about many benefits such as the improvement in inkjet recording efficiency, reduction in power consumption, prolongation of the lifetime of radiation ray generator owing to the decrease in the load, and prevention of volatilization of low-molecular substances caused by insufficient curing. Further, the increase in the sensitivity particularly increases the strength of the image formed using the inkjet recording ink. In particular, a planographic printing plate produced using the ink composition has increased strength in the image areas thereof, and thus provides higher printing durability.
In recent year, curable inkjet systems employing a radiation ray such as ultraviolet light is receiving attention because they give relatively less odor, produce fast-drying images, and allow recording on a recording medium which does not absorb ink. A cationic polymerizable ink composition which provides excellent adhesiveness to a recording medium with low shrinkage when exposed to ultraviolet light is supposed (for example, see JP-A No. 9-183928). However, the cationic polymerizable ink is not sufficiently stable during storage because of the reaction caused by the acid generated over time, which is an obstacle to the commercialization of the ink. Therefore, in order to improve the storage stability, a technique of adding a basic compound or a thermal base generator is supposed (for example, see JP-A Nos. 2003-312121, 2003-341217, and 2004-91558). However, there is another problem that the curing sensitivity of the ink deteriorates because the basic compound inhibits the function of the acid generated upon exposure.
However, in recent years, a technique of digitizing image information using a computer by electronically processing, storing, and outputting the information has become widespread, and a new image output system that can be used for the above technique has been desired. In particular, a method for making a printing plate without treatment with a developing solution is studied, and a method for directly making a planographic printing plate using an inkjet recording ink composition is studied (for example, see JP-A No. 54-117203). The method comprises steps of image-wisely ejecting ink on the support surface which is preferably hydrophilic, and curing the ink by irradiation with actinic radiation to make a printing plate having a desired image (preferably a hydrophobic image).
In order to form an image portion of a planographic printing plate, it is desired that the ink discharged onto a substrate has curability (curing sensitivity) and adhesion to the substrate. In response to such a desire, a ink containing an alicyclic polymer or a fluorine-containing polymer and a UV inkjet ink containing a resin chosen from polyurethane resin, polyamide resin, polyurea resin or polyester resin are disclosed (See, e.g., JP-A Nos. 2006-182970 and 2006-152071). However, such ink is insufficient in adhesion to a substrate, what's more it is unsatisfactory in both workability in ink preparation and ink jetting capabilities since an increase in ink viscosity during the preparation is observed.